Switching systems are used for interrupting a current or protecting an electric circuit in the event of an electrical failure for example due to a short circuit. Switching systems may comprise contacts which during normal operation are in mechanical connection. When the contacts are separated from each other a current breaking operation is effected. In addition to separating the contacts, a current breaking operation involves extinguishing an arc between the contacts, and to force the current to zero.
Alternating current (AC) switching systems utilise the naturally occurring zero-crossings of the alternating current flowing through the switching system for extinguishing the arc.
Direct current (DC) switching systems cannot utilise natural zero-crossings since there are none. It is known to create artificial zero-crossings for DC switching systems in order to be able to perform a current breaking operation. One way to obtain an artificial zero-crossing is by utilising a resonance circuit connected across the contacts. The resonance circuit comprises a capacitor which is continually charged by an energy source. The capacitor is charged to obtain a polarity which enables a capacitor discharge current to flow through the contacts in the opposite direction relative to the arc current flowing through the arc. The arrangement furthermore comprises a switch which normally is in its open state. When a current breaking operation is effected and the contacts are separated, the switch is closed, wherein the capacitor discharges its electric charge and the resonance circuit provides a current pulse into the contacts. The current pulse flows in the opposite direction relative to the arc current. By selecting suitable values of the capacitor and inductance in the resonance circuit, an artificial zero-crossing is obtained. At this time the arc generated at the contacts, which enables the arc current to continue to flow after opening of the separation of the contacts, may be extinguished by deionization of the hot plasma and/or gas in the gap between the contacts. In this manner it is possible to break the arc current.
The above-described artificial zero-crossing creation requires that the capacitor is charged at all times. Furthermore a power supply is needed to constantly charge the capacitor. Moreover, the artificial zero-crossing provides for only a single chance to successfully extinguish the arc and thus to break the arc current.